While it couldn't prove cause-and-effect, the new study found that adults who rigorously followed the so-called MIND diet faced a 53 percent lower risk for Alzheimer's, the most common type of dementia. Those sticking to the diet just "moderately well" saw their Alzheimer's risk drop by roughly 35 percent.

"Often, people who eat healthier also participate in other healthy lifestyle behavior, but the MIND diet afforded protection [against Alzheimer's] whether or not other healthy behaviors or health conditions were present," said study author Martha Clare Morris, a nutritional epidemiologist at the Rush University Medical Center and the Rush Alzheimer's Disease Center in Chicago.

The eating plan emphasizes healthy grains, vegetables, beans, poultry and fish while also allowing for a limited amount of less healthy red meat, butter and sweets.

The MIND diet combines aspects of the better-known Mediterranean diet with certain features of the so-called DASH (Dietary Approaches to Stop Hypertension) diet, both of which call for high consumption of fruits, vegetables, and fish.

But while the MIND diet stresses the importance of plant-based foods, green leafy vegetables and blueberries, it does not push much consumption of fruit, fish, dairy or potatoes.

One expert said he was intrigued by the findings.

"The protective impact they found is significant and substantial enough to make you do a little bit of a double-take," said Dr. Anton Porsteinsson, director of the Alzheimer's care, research and education program at the University of Rochester School of Medicine in New York.

"With a diet like this it seems that it's never too late to start," Porsteinsson said. "And that's a very important message."

Among the non-dietary factors Morris and colleagues accounted for were smoking history, exercise habits, educational background, mentally challenging activities (such as reading or doing crossword puzzles) and a history of obesity, depression, diabetes or heart disease.

The study results -- published in the March issue of Alzheimer's & Dementia -- suggest that the longer one follows the MIND diet, the greater the protection against Alzheimer's disease, Morris said.

Both the Mediterranean and DASH diets have been linked to a lower risk for heart disease, while some research has also identified protection against dementia.

But both diets are more stringent than the MIND diet, Morris said. The Mediterranean diet, for example, requires eating fish daily and three to four servings of both fruits and vegetables each day, she explained.

To explore how the MIND diet potentially affects Alzheimer's risk, investigators analyzed food questionnaires completed by more than 900 men and women between 58 and 98 years old. All were enrolled in the Rush Memory and Aging Project.

The surveys, completed between 2004 and 2013, set out to quantify each respondent's intake of 144 food items the prior year. No dietary intervention was involved.

Participants were then tracked for an average of four to five years, during which time they underwent repetitive neuropsychological testing.

Out of 923 adults, 144 developed Alzheimer's during that time. Those whose food consumption conformed with the MIND diet were much less likely to develop the progressive brain disorder than their peers.

Tight adherence to the DASH or Mediterranean diets also was linked to reduced risk for Alzheimer's, but loosely following either of those diets resulted in little mental benefit, the researchers found.

Morris cautioned that "this is the first study to investigate the effect of the MIND diet on Alzheimer's disease," adding more research is needed to verify the findings.

Still, the study is "well-designed and well-executed," said Porsteinsson.

He said it's noteworthy that the MIND diet seems to retain some protective benefit even if it's not strictly adhered to.

"And it may also be easier for Americans to follow, because it includes foods that are more familiar, comfortable, and available," Porsteinsson concluded.